Pressure monitor incorporating saw device

ABSTRACT

A pressure monitoring device includes a rigid base ( 2 ) which has secured thereto a lid ( 5 ) to define a hermetically sealed chamber ( 6 ). The lid ( 5 ) is of flexible metal material and the major surface ( 5 A) of the lid forms a diaphragm which will deflect in response to changes in the pressure surrounding the device. A substrate ( 7 ) upon which a plurality of SAW devices are laid down is supported on a rigid frame ( 3 A) of the base so that the central region thereof is unsupported between edges ( 11, 12 ). A dimple ( 10 ) is formed in the lid ( 5 ) and presses on a central region of the substrate ( 7 ). Changes in pressure will accordingly cause distortion of the central region of the substrate. The end regions ( 8, 9 ) of the substrate ( 7 ) carry further SAW devices so that, by analyzing the changes in characteristics of all the SAW devices a temperature compensated indication of pressure change, together with an indication of absolute temperature, may be provided by interrogating the SAW devices. The base ( 2 ) and the lid ( 5 ) are preferably both of metal alloy and may be secured together by soldering. Antennae associated with the SAW devices may be laid down as tracks on the surface of the base ( 2 ).

FIELD OF THE INVENTION

This invention relates to a pressure monitor incorporating a SAW device.

DESCRIPTION OF RELATED ART

Pressure monitors incorporating SAW devices have been proposed, forexample in our United Kingdom patent application GB-A-2352814. The SAWdevices are used to produce an electrical signal indicative of theposition of a pressure sensitive member, for example a diaphragm, whichis used to divide a chamber containing a reference pressure from achamber which is subject to a variable pressure. The present inventionrelates to an improved pressure monitor of this type.

SUMMARY OF THE INVENTION

In accordance with the first aspect of the present invention a pressuremonitor comprises: a base having a rigid frame; a lid secured to thebase to define with the base a substantially fluid tight chamber, atleast part of the lid being flexible and forming a diaphragm which willdeflect in response to changes in the fluid pressure surrounding themonitor; at least one SAW device supported within the chamber; and meansremote from the edge of the frame for transmitting movement of thediaphragm to the SAW device whereby movement of the diaphragm induced bya change in pressure in the zone surrounding the monitor will result indistortion of the SAW device.

The SAW device may be of the resonator type in which case the distortionof the SAW device may be measured by measuring the change in resonantfrequency produced by the distortion, or may be of the delay line typein which case the distortion of the SAW device may be measured bymeasuring the change in delay characteristics produced by thedistortion. Accordingly, changes in the pressure to which the monitor issubjected may be determined by observing the changes in thecharacteristics of the SAW device.

In a particularly preferred embodiment of the invention the SAW deviceis supported by the frame and extends along the length of the chamberenclosed by the rigid frame and the lid. Accordingly, deflection of thelid acting as a diaphragm will cause bending of the SAW device with aresulting change in output of the SAW device.

Preferably, three SAW resonator devices are provided having differentresonance frequencies so that by analysis of the frequency shiftsproduced by distortion of one of the SAW devices a temperaturecompensated indication of the distortion of the SAW device andaccordingly the displacement of the diaphragm may be obtained, togetherwith an indication of temperature of the SAW device. The three SAWdevices may be located on the same side of a common substrate or two maybe located on one side of a common substrate and the other located onthe opposite side of the common substrate. Alternatively, two individualsubstrates, one carrying two SAW devices (on the same or respectiveopposite sides thereof) and one carrying one SAW device, or indeed threeindividual substrates each carrying one SAW device, may be provided. Ifmore than one substrate is provided the arrangement may be such that oneSAW device is subjected to strain from the deflection of the lid and theother two SAW devices are unstrained providing references for pressureand temperature measurement.

In the preferred embodiment of the invention the means remote from theedge of the frame for transmitting movement of the diaphragm to the SAWdevice comprises one or more bumps provided on the lid which can presson the substrate of a SAW device but which cannot exert a pulling forceon the SAW device. The or each bump may be provided by a depressionformed in the material of the lid or by means of a suitable membersecured to the lid.

In one embodiment, the chamber is hermetically sealed with a referencepressure. The reference pressure is selected so that when the device islocated within the zone in which it is to operate, the pressuresurrounding the device is higher than the reference pressure with aresult that the diaphragm will deflect inwardly of the base and exert aforce on the or one of the SAW devices.

Preferably, the base is a ceramic or metal base to which the lid issecured.

In one embodiment of the invention a single sided SAW device is mountedface down supported by the base with the diaphragm bump pressing fromthe back and a second substrate is mounted in an unstrained area andcarries two reference SAW devices (one for pressure and one fortemperature). According to a further embodiment of the invention adouble sided SAW device is used to detect pressure transmitted by twobumps and a reference SAW device is mounted in an unstrained location inthe chamber for temperature reference information.

According to a second aspect of the invention a pressure monitorcomprises: a base having a rigid frame; a lid acting as a diaphragmsecured to the rigid frame to define with the base a substantially fluidtight chamber, the diaphragm having at least one indentation formedtherein to define at least one ridge projecting into the chamber; and atleast one SAW device mounted within the chamber and secured to theridge.

Preferably the lid has at least two indentations formed therein to formtwo corresponding ridges to which the substrate of a SAW device isattached. Preferably, the or each SAW device is secured to the lid by abonding process, for example using solder.

Preferably, at least two SAW devices are provided so that temperaturecompensated measures of pressure can be attained. The two SAW devicesmay be respectively mounted on one side of two substrates, or on theopposite sides of one substrate. If two substrates are provided, the lidpreferably includes three indentations whereby the two substrates mayeach be supported along one edge by a common ridge and along an oppositeedge by a respective ridge. Preferably, the substrates extend side byside along the lid.

In order to provide the desired temperature compensated indication ofpressure changes two SAW devices are preferably provided, one beingdeformed in compression and one being deformed in tension by movement ofthe lid. The SAW devices may be laid down on the opposite sides of acommon substrate or on separate substrates. If separate substrates areused they should be arranged so that simultaneously one SAW device isdeformed in compression and one SAW device is deformed in tension.

Preferably, the lid acting as a diaphragm includes a metal alloy, forexample an iron, cobalt, nickel alloy. For example the lid may be ofKovar. The base is preferably of a metal alloy (for example Kovar) butmay be of any suitable material, for example a ceramic material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following descriptionof preferred embodiments thereof, given by way of example only,reference being had to the accompanying drawings wherein:

FIG. 1A is a schematic plan view of a first embodiment of the invention,before application of the lid;

FIG. 1B is a schematic cross sectional view of the first embodiment ofthe invention along the line 1B—1B of FIG. 1A, with the lid in place;

FIG. 2A is a schematic view corresponding to FIG. 1A, but showing asecond embodiment of the invention;

FIG. 2B is a schematic cross sectional view of the second embodiment ofthe invention along the line 2B—2B of FIG. 2A, with the lid in place;

FIG. 3A is a schematic view corresponding to FIG. 1A, but showing athird embodiment of the invention;

FIG. 3B is a schematic cross sectional view of the third embodiment ofthe invention along the line 3B—3B of FIG. 3A, with the lid in place;

FIG. 4A is a schematic plan view showing a fourth embodiment of theinvention; and

FIG. 4B is a schematic cross sectional view of the fourth embodiment ofthe invention along the line 4B—4B of FIG. 4A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring firstly to FIGS. 1A and 1B, there is shown schematically apressure monitor 1 comprising a base 2 having rigid frame 3A and a rigidbase 3B which together define a shallow rectangular container 4. Thecontainer 4 may be formed of any suitable material, and in the preferredembodiment of the invention is formed of a metal material, for exampleKovar. The container 4 has secured to the open end thereof a lid 5having a major surface 5A acting as a diaphragm. The lid may be of anysuitable material, for example Kovar. Both the material from which thebase is made and the material from which the lid acting as a diaphragmis made are preferably impermeable to gas and the seal between the lidand the base is also preferably impermeable to gas. The seal between thelid in the base may be formed by any convenient means. If both the lidand the base are of suitable metal alloys the seal therebetween may beformed by a soldering process. When the lid 5 acting as a diaphragm issecured to the base 4 a sealed chamber 6 is defined by the lid and thebase. The absolute pressure contained within the chamber 6 is determinedat the time of manufacture in light of the pressure conditions to whichthe device is to be sensitive. If, for example, the device is intendedto monitor the pressure within a vehicle tire it will typically operatewithin a zone having pressure of two to ten bar and under thesecircumstances the pressure sealed within the chamber 6 may, for examplebe one bar at a temperature of 20° C. Alternatively, a vacuum may besealed in the chamber 6 to provide absolute pressure readings.

Mounted within the chamber 6 is a single substrate 7 which has formedthereon three SAW devices X, Y, Z to provide a temperature compensatedpressure and temperature monitoring output. The SAW devices may, forexample, be SAW resonators having different resonant frequencies so thatthe changes in the resonant frequencies of the respective SAW devicesmay be determined using existing conventional SAW device interrogatingtechnology.

The substrate 7 is supported by the base 2 and for convenience may besecured to the base by means of a flexible adhesive. If adhesive is usedto secure the substrate in position, the flexibility of the adhesiveshould be such that the respective end regions 8, 9 of the substrate 7are free to move as substrate 7 is acted upon by a projection 10 formedintegrally with or secured to the lid 5. Accordingly the end regions 8,9 will be substantially unstrained even when the central region isstrained as a result of movement of the diaphragm.

The lid 5 is provided, in the center of the major surface 5A, with aprojection 10 formed by a dimple pressed in the material of the lid. Theposition of the projection 10 is shown in broken line in FIG. 1A. Theprojection 10 is able to exert a downward force (as viewed in FIG. 1B)at the center of the substrate, but is unable to exert a force on ends8, 9.

Suitable antennae are provided for receiving an excitation signal foreach of the SAW devices X, Y, Z and for transmitting a response signalfrom each of the SAW devices. The antennae may be located on theexterior of the container 4 in which case appropriate electricalconnections must be provided extending through the material of thecontainer 4. Alternatively, it may be possible to lay the antennae downas tracks on the interior surface of the container 4 or on suitableareas of the substrate 7 not occupied by the SAW devices themselves.

In use, a pressure monitor as described above is placed within a zonethe pressure of which is to be monitored. The pressure monitor is chosenin light of the pressures to which it is intended to be sensitive, andin particular, the absolute pressure sealed within the chamber 6 ischosen so that the diaphragm is deflected towards the base 3B by thepressure prevailing in the region to which the device is sensitive. Thisdefection of the diaphragm will cause bending of the substrate 7 onlybetween the edges 11, 12 of the frame 3A and accordingly a change in theresonant frequency of the central SAW device Y. The resonant frequencyof the SAW devices may be determined by known techniques to calibratethe monitoring device.

If there is a change in pressure in the zone surrounding the device thiswill result in a movement of the lid 5 acting as a diaphragm. Anincrease in pressure will result in movement of the diaphragm towardsthe base 3B and a reduction in pressure will cause a movement of thediaphragm away from the base 3B. Either change will result in a changein the bending of the substrate 7 with a resultant change in theresonant frequency of the central SAW device Y. By monitoring thechanges in resonant frequency of the SAW devices an indication may beprovided of the change in pressure. The device may, for example, belocated within a vehicle tire to provide a remote indication of a lossof pressure within the vehicle tire.

The embodiment of FIG. 1 is particularly desirable in that one substrate7 is provided and accordingly SAW devices may be provided on one surfaceonly.

It will be noted that the above described embodiment of the invention isparticularly simple to fabricate and comprises in essence, only fourelements, namely the base 2, the substrate 7 (with its associated SAWdevices), the lid 5 and appropriate antennae (not shown). The componentsmay be assembled under factory conditions by means of a simple andlogical progression, namely the antennae may be laid down as suitabletracks on the base 2 and connected to the antennae, and finally the lid5 may be secured to the base to complete the unit. The simple assemblytechniques offer substantial practical advantages for a mass producedunit to be used in relatively low cost applications such as monitoringthe pressure within vehicle tires. The unit, once produced, iscompletely self-contained and merely requires to be secured in position.Securing the unit in position may be effected by semi skilled orunskilled labor within a factory environment.

In the second embodiment of FIG. 2 two substrates 20, 21 are providedinstead of the single substrate 7 of FIG. 1, and at least two SAWdevices are provided on the lower unstrained substrate 21. In otherrespects, the components in operation of the embodiment of FIG. 2 aresubstantially the same as those of the embodiment of FIG. 1. As in thecase of the embodiment of FIG. 1, the substrates 20, 21 may convenientlybe held in position by means of flexible adhesive.

In the third embodiment (FIG. 3) two substrates 30, 31 are provided andtwo SAW devices are provided on opposite sides of the upper strainedsubstrate 30. In other respects, the components in operation of theembodiment of FIG. 3 are the same as those of the embodiment of FIG. 1.

Referring now to FIGS. 4, a further embodiment of the invention isshown. As in the case of the embodiment of FIGS. 1-3 the devicecomprises a container 4 which is of ceramic or metal material. Thecontainer 4 comprises a rigid frame 3A and a base B. The open mouth ofthe container is closed by a lid 5 acting as a diaphragm to define ahermetically sealed chamber 6. In the case of this embodiment a SAWdevice substrate 40 to detect changes in deflection of the diaphragm 5is secured to the diaphragm itself. To this end, pressed indentations42, 43 are provided in the diaphragm. The indentations 42, 43 defineridges within the chamber 6 to which the substrate 40 is secured, forexample by soldering. The substrate 40 has mounted thereon one or moreSAW devices. A further substrate 41 is mounted on the unstrained area ofthe chamber 6 As will be appreciated by those skilled in the art, atleast two SAW devices are required to provide a pressure indicationwhich is temperature compensated and three SAW devices are required toprovide pressure and temperature monitoring capabilities.

In the case of embodiments of FIGS. 4, deformation of the lid 5 actingas a diaphragm caused by pressure changes will result directly indeformation of the substrate, 40 with a resulting change in the resonantfrequency of the SAW devices mounted thereon. Antennae for the SAWdevices may be provided by any convenient means, for exampleelectrically conducted track provided on the surface of the container 4or electrical connections to external antennae passing through thecontainer 4.

1. A pressure monitor comprising a base having a rigid frame; a lidsecured to the base to define with the base a substantially fluid tightchamber, at least part of the lid being flexible and forming a diaphragmwhich will deflect in response to changes in the fluid pressuresurrounding the monitor, at least one surface acoustic wave (SAW) devicesupported within the chamber; an means remote from the edge of the framefor transmitting movement of the diaphragm to the SAW device wherebymovement of the diaphragm induced by a change in pressure in a zonesurrounding the monitor will result in distortion of the SAW device; andthe SAW device comprising surface elements laid down on a substrate, thesubstrate having, in addition to the zone thereof which is subject todistortion by movement of the diaphragm, at least one other region whichis not subject to substantial distortion as a result of movement of thediaphragm, and upon which at least one further set of surface elementsare laid down to form at least one further SAW device to providetemperature compensation to the output of the monitor.
 2. A pressuremonitor according to claim 1 wherein three SAW devices are providedwhereby the monitor provides a temperature compensated indication ofpressure changes, together with an indication of the temperature of themonitor.
 3. A pressure monitor according to claim 1 wherein the meansfor transmitting movement of the diaphragm to the SAW device comprises aprojection extending from a major surface of the lid.
 4. A pressuremonitor according to claim 3 wherein the projection is formed bydeforming material of the lid.
 5. A pressure monitor according to claim1 wherein at least one antenna for providing radio communication to theSAW device is laid down on a surface of the base.
 6. A pressure monitoraccording to claim 1 wherein the lid is of a metal or metal alloy and issecured to the base by soldering.
 7. A pressure monitor according toclaim 1 wherein the chamber is hermetically sealed and includes a gas ata pre-determined absolute pressure.
 8. A pressure monitor according toclaim 1 including a plurality of SAW devices provided on a plurality ofsubstrates, at least one of the substrates being secured to the base ina manner which is not the subject to distortion as a result of movementof the diaphragm.
 9. A pressure monitor according to claim 1 wherein thesubstrate of the at least one SAW device is secured to the base by meansof a flexible adhesive.
 10. A pressure monitor comprising a base havinga rigid frame; a lid acting as a diaphragm secured to the rigid frame todefine with the base a substantially fluid tight chamber, the diaphragmhaving at least one indentation formed therein to define at least oneridge projecting into the chamber; and at least one SAW device mountedwithin the chamber and secured to the ridge, wherein the at least oneSAW device includes three SAW devices whereby the monitor provides atemperature compensated indication of pressure changes, together with anindication of the temperature of the monitor.
 11. A pressure monitoraccording to claim 10 comprising a substantially fluid tight chamberhaving a lid forming a diaphragm which deflects in response to changesin the fluid pressure surrounding the monitor; the at least one surfaceacoustic wave (SAW) device supported within the chamber; and means fortransmitting movement of the diaphragm to the at least one SAW devicewhereby movement of the diaphragm induced by a change in pressure in azone surrounding the monitor results in distortion of the at least oneSAW device.
 12. A pressure monitor according to claim 10 wherein the atleast one SAW device comprises surface elements laid down on asubstrate, the substrate having at least one region which is not subjectto substantial distortion as a result of movement of the diaphragm, andupon which at least one further set of surface elements are laid down toform at least one of the SAW devices to provide temperature compensationto the output of the monitor.
 13. A pressure monitor according to claim12, wherein the plurality of SAW devices are provided on a plurality ofsubstrates and includes at least one of the substrates being secured ina manner which is not the subject to distortion as a result of movementof the diaphragm.
 14. A pressure monitor according to claim 12 whereinthe substrate of the at least one SAW device is secured to the base bymeans of a flexible adhesive.
 15. A pressure monitor according to claim11 wherein the means for transmitting movement of the diaphragm to theat least one SAW device comprises a projection extending from a majorsurface of the lid.
 16. A pressure monitor according to claim 15 whereinthe projection is formed by deforming material of the lid.
 17. Apressure monitor according to claim 10 wherein the lid is of a metal ormetal alloy and is secured to the base by soldering.
 18. A pressuremonitor according to claim 10 wherein the chamber is hermetically sealedand includes a gas at a pre-determined absolute pressure.
 19. A pressuremonitor comprising: a base having a rigid frame; a lid secured to thebase to define with the base a substantially fluid tight chamber, atleast part of the lid being flexible and forming a diaphragm which willdeflect in response to changes in the fluid pressure surrounding themonitor; a plurality of surface acoustic wave (SAW) devices provided ona plurality of substrates supported within the chamber; means remotefrom the edge of the frame for transmitting movement of the diaphragm tothe SAW device whereby movement of the diaphragm induced by a change inpressure in a zone surrounding the monitor will result in distortion ofat least one the SAW devices; and at least one of the substrates beingsecured to the base in a manner which is not the subject to distortionas a result of movement of the diaphragm.